1. Field of the Invention
The present invention concerns a 3D image post-processing method of the type wherein an x-ray contrast agent is administered via a micro-catheter (catheter angiography), as well as a medical-technical apparatus for three-dimensional representation of sections of the blood vessel system of a patient that, in particular in the field of coronary angiography, can be used to diagnose diseases of the coronary vessels, as well as in the field of neuro-angiography for (pseudo-) three-dimensional vascular imaging in the framework of an endovascular intervention.
2. Description of the Prior Art
For preventative treatment of coronary and cerebral aneurysms, angiomas and other arterial-venous malformations (AVM), endovascular interventions are ever more frequently employed today as an alternative to coronary or neurosurgical procedures. In the preliminary stages of an endovascular intervention, a contrast agent-intensified angiographic examination of the blood vessel sections affected by a vessel anomaly is normally implemented.
In catheter angiography, the blood vessels to be examined are made visible by x-ray exposures after injection of a contrast agent. For example, a micro-catheter of approximately 1.5 mm external diameter, inserted intravascularly via the inguinal artery, is moved to the blood vessels to be examined. Since this is a minimally-invasive method, catheter angiography is today used only seldomly for purely diagnostic purposes. For example, using modem micro-catheters (which typically have an external diameter of approximately 0.6 mm), in stroke patients the location of a vessel narrowing (stenosis) caused by arteriosclerosis or a blood clot can be found inside the skull and, for example, thromboses can be loosened in an endovascular interventional procedure with the aid of a fibrinolytic medicine.
The intravascular insertion of a micro-catheter for endovascular therapy of a blood vessel deformation, as well as the navigation of this catheter within the blood vessel system to be treated, is a difficult and demanding procedure, since the catheter must be guided through a number of vessel branchings. This procedure can be significantly simplified by a 3D visualization of the sections of the vessel system to be examined. Conventionally, the navigation of the catheter through the vessel tree is monitored fluoroscopically. In order to show the blood vessels, contrast agent is thereby applied by the tip of an intravascularly inserted micro-catheter in order to make the blood vessels in the environment of the catheter tip visible in the x-ray image. In order to reconstruct the three-dimensional structure of the vessel tree, a number of x-ray images are necessary from various directions. Afterwards the catheter is moved further forward without the imaging method being continued during this movement.
Systems for angio-cardiography are specially designed for the display of heart chambers, heart valves and heart vessels. They must satisfy all requirements for the diagnosis and must support the implementation of the appropriate therapeutic procedures that are possible with the procedures of interventional cardiology. Among these procedures are, for example, vessel dilation as well as the expansion of the heart valve cross-section using balloon catheters, thrombolysis with pharmaceuticals applied via catheter directly at the location of a lesion, and the opening of blocked coronary arteries by laser beams.
Since the risk for the patient increases with the duration of the examination or procedure, for the patient's interest, as well as for economic reasons, an invasive examination of the heart should be implemented in a manner that saves as much time as possible. The same is true for therapeutic procedures within interventional cardiology. An imaging system for this purpose should optimally allow simultaneous imaging in two different projection directions (biplanar operation), or should allow a rapid change (switching) between two projection directions. In the exposure, a biplanar system significantly eases the orientation during the positioning of the catheter and reduces the need of contrast agent in the exposures and the retention period of the catheter in the vessel, since two projection directions can be acquired at the same time with a single injection. At the same time, a quantitative description or classification of existing stenoses is simplified by the simultaneous biplanar representation.
The coronary vessels are spread over the curved heart surface, and therefore can be represented only from a single projection direction without distortions in sections. For this reason, and for the elimination of overlappings of individual vessels, a number of different projections are necessary for a complete examination of the coronary system.
Digital subtraction angiography (DSA), computed tomography angiography (CTA), magnetic resonance or nuclear magnetic resonance angiography (MRA) and ultrasound diagnosis methods (for example Doppler sonography and color-coded duplex sonography) (CCDS) are available today for the diagnosis of blood vessel diseases. Primarily, developments in the field of computed tomography (CT), magnetic resonance tomography (MRT) and ultrasound diagnostics have clearly limited the need for invasive examination methods (for example, DSA) in the framework of the vessel diagnosis. It can be expected of the newest developments, in particular in the field of MR angiography and ultrasound diagnostics, that in the near future the primary vessel diagnostics will be implemented almost completely with non-invasive, more patient-friendly examination methods.
In the following, a brief description is provided for the most important examination methods used in the framework of a preoperative endovascular intervention, knowledge of which is necessary for the comprehension of the present invention (DSA, CTA, MRA and computer-aided 3D rotation angiography).
Digital subtraction angiography (DSA) is an invasive examination method that today is accepted ever more as the radiological standard method for diagnosis of vessel diseases—in spite of large advances in the field of CT and MR angiography. It requires an arterial puncture and the insertion of a catheter into the concerned examination region, the administration of an x-ray contrast agent (normally containing iodine), and a relatively high radiation exposure. An advantage is the (in comparison to other imaging methods) comparably high spatial and temporal resolution. Only the perfused lumen of a vessel, however, can be shown with DSA. Moreover, with this examination method vessel narrowings (stenoses) caused by atherosclerotic deposits (plaque formation) on the inner walls cannot be verified, or can be verified only indirectly. The use of invasive DSA consequently is used only when an interventional measure (such as, for example, a balloon dilation or stent introduction) is additionally planned, for the vessel representation.
In DSA, two x-ray exposures are produced at a temporal interval from one and at the same viewing angle: an exposure of the blood vessels not yet filled with contrast agent (what is known as the mask image) and an exposure of the vessel filled with an x-ray contrast agent after the contrast agent injection (the fill image). The x-ray images are then subtracted from one another, so congruent image portions of bones and soft parts are eliminated such that only the blood vessels filled with the x-ray contrast agent are shown. Typically, before the subtraction a logarithmization of the intensity signals corresponding to the fill and mask images is done, such that the obtained difference image is directly proportional to the contrast agent concentration and all tissue structures outside of the blood vessels to be examined are eliminated by the subtraction.
Computed tomographic angiography (CTA) is a minimally-invasive examination method with a high spatial resolution but a comparably limited temporal resolution, for which, by spiral computed tomography, a (pseudo-) three-dimensional representation of the larger blood vessels is generated. Similar to DSA, the method requires the administration of contrast agent (normally containing iodine) and a non-negligible radiation exposure. A significant diagnostic advantage of CTA, however, is that it allows detection of atherosclerotic plaque, as well as the possibility to assess the wall structure of larger vessels, and the exact determination of the lumen width of a vessel. In comparison to DSA, the CTA offers a better diagnostic conclusion in the preliminary stage of percutaneous endovascular interventions. Conventional reconstruction methods enable a 3D representation of larger vessels, but require a relatively long post-processing time for 3D reconstruction from x-ray images acquired from various projection directions.
Magnetic resonance imaging or magnetic resonance angiography (MRA) is a non-invasive examination method with very good spatial and temporal resolution and a relatively high contrast resolution. Improvements in the diagnostic significance, above all in the area of smaller vessels, can be achieved by the increasing use of paramagnetic contrast agents. Similar to computer tomography, magnetic resonance tomography allows both the perfused lumen and changes of the vessel walls to be shown. Due to the higher contrast resolution, however, it is clearly superior to computed tomography. Image post processing and 3D reconstruction of large vessel sections are possible in an extremely short time.
In order to improve the topographic representation of the vessel tree, optical realization of the third dimension in angiographic studies has been studied since the 1970s. Rotation x-ray examinations of the brain where first introduced in clinical practice, wherein all auxiliary projections were achieved by rotation of an x-ray tube around the head of a patient, and thus stereoscopic views of the cerebral blood vessels were possible. The introduction of computer-aided rotation angiography brought the technical breakthrough to reconstruct the (pseudo-) three-dimensional representations with isotropic resolution from the raw projection data, as is described in the article “Use of a C-Arm System to Generate True Three-Dimensional Computed Rotational Angiograms: Preliminary In-Vitro and In-Vivo Results” (AJNR 1997, Vol. 18, pp. 1507 through 1514) by R. Fahrig, among others, 3D rotation angiography has proven to be a valuable complement to conventional angiographic examination methods, both in diagnostic and therapeutic procedures. From the spatial information that is acquired at a workstation in the interactive evaluation of 3D reconstructions of x-ray images acquired from a number of projection directions, 3D rotation angiography is far superior to a conventional digital biplanar subtraction angiography in the assessment of neck aneurysms and the relation to adjacent blood vessels. The 3D reconstructions of the vessel structure are graphically visualized using a volume rendering technique (VRT), multiplanar reformatting (MPR) or maximal intensity projections (MIP). Moreover, they can be rotated and thus considered from various perspectives. Due to the high speed of the post-processing software, 3D angiography has today developed into a tool that can be directly used in the planning and implementation of endovascular interventions.
In the last few years, further improvements have been made to the imaging systems and the software to generate 3D images. Meanwhile, 3D imaging systems are available on the market from various angiography apparatus manufacturers. Given patients on whom DSA examinations have been implemented with faster image reconstruction and high image quality, 3D angiography has proven to be more advantageous than the standard DSA in the evaluation and treatment planning of intracranial aneurysms. In contrast to rotation angiography, an image can be viewed from an arbitrary perspective. With regard to arterial stenoses, the three-dimensional image can expose anomalies or the degree of a lesion that cannot be determined in planar views. Any arbitrary perspective is possible, including viewing angles that would be impossible with DSA due to the limited mobility of the angiographic apparatuses.